1. Field of the Invention
The invention relates generally to an objective lens system ("objective") for use in endoscopes and the like, and more specifically to landscape and retrofocus type objectives for endoscopes.
2. Description of the Prior Art
Endoscopy is a technique that allows the examination of an area of a body by means of an endoscope, which is a tube like instrument with lenses and a light source attached. The endoscope provides a visual examination of the interior of a body through a natural body opening such as the throat, or through a small incision into the body. Because an endoscope can be inserted through a very small opening, it is a less invasive method than other surgical techniques, causing less scaring and quicker recovery time. A camera or video recorder is often used during an endoscopic procedure to provide long term records of internal organs which can be used for later reference. The increasing use of endoscopes for both diagnostic and therapeutic uses has substantially reduced the need for invasive surgical procedures such as gynecological and reproductive disorders, biopsies, gastrointestinal track inspection, knee and other joint surgery and the like.
In basic structure, the endoscope typically includes an ocular at its proximal end and an objective assembly at its distal end. Between the ocular and the objective assembly, a typical endoscope will include elongated relay optics of relatively small cross-section. The relay optics can be rigid and include rod lenses spaced along a sheath. Alternatively, the relay optics can be flexible and include optical fibers.
The smaller the cross-section of the distal end of the endoscope, the less invasive the procedure. For medical applications, these optics have to be relatively good and without visible distortion. There is a tension between reducing the cross-section of the endoscope distal end and improving its optical quality. The resulting conventional optical systems used in endoscopes are generally complex and employ various combinations of lenses within the elongated tube or sheath, typically including the objective lens, the relay lens system, and the ocular or eyepiece. The objective lens forms an image at the inner distal end of the tube which is then picked up and transmitted by the relay lens system to the proximal end of the tube where it is viewed with the ocular.
The objective lens is generally a wide angle lens having a short focal length and is physically separated from its environment by a rugged, transparent optical material. In order to correct for the various chromatic as well as geometric aberrations inherent in lenses, it is generally necessary to employ several different lenses, each of which corrects or partially corrects for aberrations in the optical train. Geometric aberrations are typically reduced by increasing the number of lenses or aspherizing one or more surfaces, whereas chromatic aberration can be corrected for by the proper choice of low and high dispersion glasses. Typical prior art endoscope designs include retrofocus lens systems made of glass having one or more aspheric surfaces. (See, for example, U.S. Pat. Nos. 4,403,837 to Nakahasi and 4,867,546 to Nishioka et al.) Unfortunately, these systems are relatively complicated, employing aspheric lenses, and typically have relatively long total tracks and higher than desirable distortion.
The use of sapphire as a material in endoscopic systems has been largely limited to that of a protective covering or window at the distal end of the endoscope. A sapphire barrier at the distal end of an endoscope is said to have enhanced resistance to the higher temperatures and pressures associated with sterilization procedures, such as found in an autoclave. U.S. Pat. No. 5,377,669 to Schulz discloses a sapphire protective covering whose proximal end is so constructed that the sapphire also functions as a negative lens. Although this sapphire negative lens increases the field of view of the objective, following nonsapphire optics are used to correct for geometric and chromatic aberrations. U.S. Pat. No. 5,555,131 to Horton describes an objective that employs a sapphire positive lens/plastic negative lens combination immediately following a virtual image of the stop. The other elements in the optical train are plastic with low indices of refraction and moderate dispersion. Since the refractive index of plastic is relatively low, an endoscope in accordance with the Horton patent has a relatively small field of view and a high f-number. To compensate for plastic's low index of refraction, aspheric surfaces are required, but they do not have enough bending power to limit the lenses to a small diameter, which results in the endoscopic procedure being more invasive in a surgical procedure. U.S. Pat. No. 5,424,877 to Tsuyuki et al. discloses an observation optical system for endoscopes in which the objective lens system may include a sapphire negative lens element for obtaining a wide angle visual field. Sapphire is also used in rod form in fiber optic probe systems (U.S. Pat. No. 5,166,756 to McGee et al. and U.S. Pat. No. 5,351,322 to VonBargen) and as a contact member in a surgical laser probe (U.S. Pat. No. 4,592,353 to Daikuzono).
There is still a need, however, for endoscopic systems, and in particular objective lens systems for endoscopes and the like, that are compact, small in diameter, simple, and offer the user high optical performance.